In the process of sensing light with photosensitive semiconductor devices, usually in some array, switching and amplifying circuits are required to process the electrical signal representing the light sensed. The switching and amplifying circuits, however, introduce Nyquist noise during their operation to degrade the sensitivity of the sensor device and consequently the usuable information detected therefrom. The nature of these adverse effects are described by White et al. in their article "Characterization of Surface Channel CCD Image Arrays at Low Light Levels," Journal of Solid-State Circuits, Vol. SC-9, No. 1, Feb. 1974. In this article, the authors describe the method of "second correlated sampling" to minimize these adverse effects. This process is also described in U.S. Pat. No. 3,781,574, issued to White et al. and entitled "Coherent Sampled Readout Circuit and Signal Processor for a Charge Coupled Device Array," herein incorporated by reference.
FIG. 1 shows a simplified charge-coupled device (CCD) output structure using second correlated sampling for noise reduction. The circuit operates as follows, where I.sub.S represents the pulse of current resulting from a packet of electrons emanating from the CCD. The output signal from a CCD shift register is the packet of electrons which is collected and stored on capacitor C1. To remove the signal charge from C1 in preparation for the next cycle, switch S1 is closed and capacitor C1 is charged to a reference voltage V.sub.R1. The thermal noise associated with the finite resistance of switch S1 causes a random variation in the amount of charge on capacitor C1. When switch S1 is opened, a last value of this random charge is stored on capacitor C1. The root-mean-square value of the total spectral noise charge is .sqroot.KTC1, where K is Boltzmann's constant and T is absolute temperature. The root-mean-square voltage is .sqroot.(KT/C1). After switch S1 opens, a second switch S2 is closed. This charges the input of an amplifier A1 to a second reference voltage V.sub.R2. The source for voltage V.sub.R2 is a low impedance supply. During this period of time, a capacitor C2 stores the noise signal on its left plate while its right plate remains at V.sub.R2. This, of course, is accomplished by amplifier A1 having a gain of unity. Switch S2 is then opened, and the circuit is ready to accept the signal packet from the shift register. The signal is a.c.-coupled onto the right plate of capacitor C2 with no component due to the thermal reset noise. Therefore, the thermal noise of S1 has effectively been eliminated.
In the circuit of White et al. in U.S. Pat. No. 3,781,574, shown herein in FIG. 2, second correlated sampling is accomplished in the manner just described. This prior art circuit, however, has several disadvantages associated with it. One is the extra power supply required to provide the second reference voltage V.sub.R2. The power supply has to have a constant output and must have a low output impedance in order to quickly charge capacitor C1 with the thermal noise voltage. Another disadvantage is that the second reference voltage to which node N is clamped (C.sub.N) does not track with process variation in the semiconductor fabrication of the semiconductor circuit. Thus, if the threshold voltage of transistors within the amplifier connected to the second reference voltage varies, so will the amplifier characteristics. The varying bias conditions causes concomitant fluctuations in gain and frequency response. In the prior art, a typical solution is to use a variable resistor to make adjustments to V.sub.R2 in each circuit to account for the varying bias conditions. Not only does this solution preclude a simple integrated circuit implementation of a second correlated sampling circuit, it also necessitates troublesome adjustment of individual circuits. Still another disadvantage to this solution is the relative complexity of the circuit; a complex circuit comprising discrete elements tends to be more failure-prone than a simpler circuit comprising integrated elements.
In the present invention, the circuit in accordance with the preferred embodiment overcomes all the above disadvantages. By using negative feedback, this novel circuit dispenses with the requirement of a separate reference power supply for voltage V.sub.R2. The negative feedback in the novel circuit further insures that bias conditions of the gain stage following the second correlated sampling section remains constant in spite of process deviations during the course of manufacturing. Individual adjustment of the circuit is also dispensed with. Furthermore, the circuit in accordance with the preferred embodiment has fewer circuit elements, such as resistors, which are difficult to fabricate in an integrated circuit. All these advantages in the novel circuit combine to make it fully integratable, thus resulting in a higher performing and more dependable circuit.